Kafka: At-least-once means your consumer must be idempotent

@KafkaListener(topics = "payments")
void onPayment(PaymentEvent e) {
    // Natural idempotency key from the event
    if (processed.putIfAbsent(e.paymentId(), TRUE) != null) return;
    ledger.credit(e.account(), e.amount());
}

// Better: a unique constraint in the database
// insert into processed_events(event_id) values (?)  -> duplicate key = skip

A rebalance mid-batch redelivers the last few records.
Without a guard, the account is credited twice.